The present invention relates to the field of gas chromatography. More particularly, the present invention relates to a method and apparatus for automatically analyzing organic elements of a sample from a continuously flowing liquid stream. In one aspect, the present invention relates to an on-line or automated gas chromatographic system for analyzing trace polar organic contaminants in a water process stream for use in electronics and semiconductor manufacturing operations.
Generally, when various solutions which are used in the manufacturing processes of many diverse industrial fields are to be subjected to an ingredient analysis for the purpose of quality or process control, the particular solution is sampled, and carried to the place where the analyzing apparatus is located. This analysis can be time-consuming and insufficiently responsive to provide timely information to control the process stream. Alternatively, on-line analysis of the chemical composition of one or more solutes in a carrier medium (e.g., a solvent) may be accomplished. A number of different analysis techniques can be utilized, including gas chromatography, to determine the nature of solutes in the process stream.
For example, in the manufacturing of semiconductor devices, ultra-pure water is used as a cleaning solvent. Organic contamination of this water, which occurs during routine operations such as wafer rinsing, affects the ability of the manufacturer to reuse the water. Currently, due to insufficient on-line monitoring methods, this process water is commonly diverted to waste or large holding tanks until adequate chemical analysis can be performed. To conserve the resource and subsequently reduce operating costs, this water can be reused in the same process or recycled for other applications, such as industrial cooling. However, for these applications, adequate on-line monitoring methods must be available to identify contaminants that can upset processes or damage equipment. Rapid detection of contamination, generally at sub part per million by volume (ppm.sub.v) or low part per billion by volume (ppb.sub.v) levels, is essential to prevent a decrease in product yield. Commercially available process monitors based on oxidation-CO.sub.2 or combustion-ion chromatography methods are limited to sampling times of greater than three minutes. These monitors measure "total organic carbon" and have low part per billion (ppb.sub.v) detection limits, although they do not identify the individual analytes present.
Gas chromatography is one of the most popular methods for sample analysis. A gas chromatography apparatus includes a sample injector for sample thermal vaporization and transfer into a separation column, a separation column in a temperature-controlled oven and a suitable detector to record the amount and time of appearance of the analyzed compounds. The injected sample mixture, comprising a solvent with a number of solutes, is separated in time and each solute is identified by its time of elution. The time of elution for each solute is a characteristic of its chemical and physical properties relative to the solvent and other solutes.
To detect and analyze volatile organic compounds, the compounds are typically extracted from aqueous solutions by bubbling a sparging gas through the liquid. The analyte comes to an equilibrium concentration in the sparging gas, and the gas is then analyzed using gas chromatography. Sparging is a process by which volatiles are extracted from water by bubbling a clean gas through the water; however, this process generally takes several minutes. The sparging gas then flows through the gas chromatographic injection system for separation via a separation column (e.g., a capillary column) and a detection device. The separation column operation can be optimized for the individual constituents that are to be detected by varying the column temperature, type, and physical characteristics. The overall analysis time ranges from approximately 10 to 20 minutes, depending on the nature of the constituents to be detected.
Generally, samples are introduced into the gas chromatograph (GC) through injection from a small syringe that penetrates a septum and dispenses a measured amount of liquid sample into the GC injector for vaporization and further transport by a carrier gas into a separation column. The effluent from the separation column is passed through a detection device. Typical GC detectors include a flame ionization detector (FID), a flame photometric detector (FPD), a thermionic detector (TID), a thermal conductivity detector (TCD), an electron captive detector (ECD), a mass spectrometer (MS), or a Fourier transform infrared spectrometer (FTIR).
The effectiveness of a GC to detect particular solutes depends on the relative characteristics of the solutes to the carrier solvent as well as the characteristics of the GC separation column and detector. In general, solutes that are closely related by chemical characteristics to the solvent are more difficult to separate and generally require longer analysis times. For example, polar solutes with water as the solvent are difficult to separate because the solutes may be extremely soluble in the polar water solvent. In semiconductor process operations, such polar solutes consist of such compounds as methyl ethyl ketone (MEK), isopropanol (IPA), acetone, p-xylene, butyl acetate, ethyl benzene, ethylene glycol, and 2-ethoxyethyl acetate. A typical separation column can be several tens of meters in length, requiring minutes of transport time for the sample to pass through the column and detector.
Current methods for preconcentration of the solutes to be analyzed by gas chromatography analysis include membrane separation, sorbent trapping or extraction, cryo-trapping, and liquid--liquid extraction. All of these preconcentration methods significantly increase the time of analysis, generally requiring several to tens of minutes. Mitra (U.S. Pat. No. 5,435,169, issued on Jul. 25, 1995) discloses a method and device for continuous monitoring of low level concentrations of volatile organic compounds in waste water streams, where the fluid stream passes through an on-line micro sorbent trap to concentrate the organic compounds. This sorbent trap contains the sample for a period of time before releasing it as a desorption pulse for subsequent contaminant monitoring. The sorbent trap is designed to retain and concentrate the analytes of interest; therefore, the trap must have an adsorbent specific to the analytes of interest that will not react with the solvent. Amirav et al. (U.S. Pat. No. 5,686,656, issued on Nov. 11, 1997) describe a method and device for introducing liquid samples into a gas chromatograph, particularly samples containing biological compounds. The method, however, requires several minutes for analysis and does not allow for on-line and continuous sample analysis.
Direct injection methods, which use an automated valve to sample and inject a portion of the liquid stream into a gas chromatograph, have the potential to be much faster. The sampling rate of automated systems is dependent only upon the time required for analyte separation in the chromatographic column. Due to the high volumetric flow rates commonly encountered in process-streams, rapid sampling and analysis rates improve the accuracy of the chemical contaminant information and the ability to define the actual volume of the process water being analyzed.
Filippini et al. (Analytica Chimica Acta, 255, 91-96, 1991) describe an analysis method for on-line capillary gas chromatography with automated liquid sampling using a liquid sample injection valve that vaporizes the liquid sample by means of a heated carrier gas. In Filippini et al., the injection valve is attached to the commercial gas chromatograph, which must be heated to high temperature (220.degree. C.), as this is where the vaporization of the liquid occurs. This causes injection variability due to potential bubble formation in the metering groove, as well as buildup of non-volatile contaminants within the metering groove.
Cortes et al. (U.S. Pat. No. 5,522,988 issued on Jun. 4, 1996) describes an on-line coupled liquid chromatography and gas chromatography apparatus. The apparatus includes a vaporizing chamber that is interposed between a liquid chromatographic column and a capillary gas chromatographic column. Because of the use of liquid chromatography, the input stream to the liquid chromatographic column cannot be completely aqueous. Additionally, the analysis requires at least several minutes to complete.
In spite of the above, sampling and analysis of industrial water recycling, wastewater, or process streams present many challenges. For many water recycling and wastewater processes, particularly water process streams used in the electronics and semiconductor industries, the stream must be sampled on line and in a continuous or near-continuous manner. The analysis must be rapid to minimize potential subsequent contamination downstream. The analysis must also be able to detect low concentrations, generally at sub part per million levels, of organic contaminants.
The automated, aqueous, gas chromatographic system of the present invention solves many problems encountered in traditional recycling stream analysis. Typically, the recycling/process stream is sampled periodically, followed by off-line analysis. The present invention, relative to off-line analysis, decreases the chance of human error, eliminates the need for external handling and therefore the possibility of external contamination, and significantly reduces the time required for analysis. Existing systems employ holding tanks to temporarily store effluent while off-line analysis is performed, at which time the tank contents are then sent to waste or back into the process stream. Also, a low-volume contamination event can flow into a (potentially uncontaminated) holding tank, resulting in a much larger volume that must then be sent to waste. The automated on-line stream analysis apparatus and method of the present invention allows the divert decision to be made rapidly and automatically, eliminating the need for holding tanks and human supervision. This increases the efficiency of the recycle process, reduces waste production and the labor required, all of which reduce the costs associated with process stream recycling. Efficient recycling is of great importance, for example, in the semiconductor industry, which uses highly purified water in many processes. Purifying the water is initially costly, and re-use of impure recycled water can result in product loss and production downtime.
Ideally, process streams must be monitored continuously, reliably, inexpensively, and with minimum supervision. The present invention utilizes inexpensive, rugged parts already commercially available and allows measurements to be made in less than one minute. The unit is fully automated, requires little supervision, and performs alarm functions under user-defined conditions of contaminant detection.
The present invention has several advantages relative to existing on-line gas chromatographic technologies. No analyte sparging is required and, except for the brief sampling time, carrier gas flows continuously through the same valve that introduces the sample. The vaporization column connects directly to the valve and only a small aliquot sample is taken from the process stream. Common auto-sampler or auto-injector parts such as needle, septum, and glass injection liner have been eliminated and each sub-system is entirely isothermal. The present invention provides a system to control the process stream and allow decisions to be made to divert the liquid in the process stream to waste in near real-time, eliminating the need and therefore cost for holding tanks required for storage while waiting for off-line analysis.
Relative to on-line "total organic carbon" (TOC) analyzers, the present invention has a much shorter response time, one minute or less for common polar hydrocarbons. This allows contamination events to be defined more precisely, in turn preventing waste of uncontaminated water and reducing the spread of the contaminant. Because TOC analyzers add chemical reagents to the water, requiring that the flow to the instrument be sent to waste without fail, the present invention uses a smaller aliquot, and consumes the sample during analysis. For the present invention, wastewater is only created during a calibration event. TOC analyzers give a single measurement value, whereas the present invention allows the user to quantify and identify individual contaminants. This information may allow the user to determine the source or location of the process anomaly, which is not possible using a TOC analyzer. The present invention also has the flexibility to be customized for a particular contaminant or group of contaminants. Different column coatings, commercially available, can be interchanged to customize the instrument.